Studying black holes is a tricky business, but not always because they're so dim – to the contrary, in some cases it's hard to see through the bright cloud of hot gas that surrounds them. Now, a team of astronomers has used a global array of telescopes to peer through that fog and capture some of the clearest radio images yet of Sagittarius A*, the supermassive black hole at the center of the Milky Way. And it may indicate that the object just happens to be pointing directly at Earth.
About 26,000 light years away, Sagittarius A* is probably not the closest black hole to Earth, but it is the closest supermassive one, with the mass of around 4 million Suns. That makes it the ideal target for astronomers looking to snap an image of a black hole. Of course, since light itself can't escape the object's gravitational pull, there's not much to see on its own, but the goal is to see the "black hole shadow" – the silhouette of the object framed against the bright background of matter falling into the black hole.
But the problem is that since there's half a galaxy between it and us, the light is scattered by all the matter in our line of sight, creating a bright cloud that obscures Sagittarius A*. To see through that, astronomers have now essentially used a virtual telescope the size of Earth.
This technique is known as very-long-baseline interferometry (VLBI). It's performed by pointing several different radio telescopes at the same object at the same time, then comparing the differences between when the signals from the source hit each of the telescopes. That data can then be processed to remove most of the scattering effect, producing a clearer radio image of the object.
Astronomers used the Global Millimeter VLBI Array (GMVA) to do just that, observing Sagittarius A* at a frequency of 86 GHz. But the key development is that this was the first time the Atacama Large Millimeter Array (ALMA) telescope in Chile was used as one of 13 telescopes in the GMVA, which is particularly sensitive at this frequency. The end result was an image with twice the resolution of previous attempts at 86 GHz.
The study revealed a few new details about the supermassive black hole. The source of the radio emissions was found to be symmetrical, and most of the signals are coming from one area much smaller than previously thought – covering just one 300 millionth of a degree in the sky. That surprise may change our understanding of what exactly is giving off the signals – or that Earth has lucked into a very specific position to view it from.
"This may indicate that the radio emission is produced in a disk of infalling gas rather than by a radio jet," says Sara Issaoun, lead researcher on the project. "However, that would make Sagittarius A* an exception compared to other radio-emitting black holes. The alternative could be that the radio jet is pointing almost at us."
In future, an even more sensitive telescope array, known as the Event Horizon Telescope, is planning to study Sagittarius A* at a frequency of 230 GHz. That should return even higher resolution images of the supermassive black hole.
"Even though scattering blurs and distorts the image of Sagittarius A*, the incredible resolution of these observations allowed us to pin down the exact properties of the scattering," says Michael Johnson, co-author of the study. "We could then remove most of the effects from scattering and begin to see what things look like near the black hole. The great news is that these observations show that scattering will not prevent the Event Horizon Telescope from seeing a black hole shadow at 230 GHz, if there's one to be seen."
The research was published in The Astrophysical Journal.
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